We have continued and expanded our analysis of the phenomenon of swelling of the nerve fiber during excitation. (This phenomenon was discovered in this Laboratory about two years ago.) After establishing the fact that the degree of swelling reaches its maximal level at the peak of the internally recorded action potential in the squid axon, the following four types of investigation were carried out: (1) determination of the time-course of rapid mechanical responses of the squid axon treated with tetraethylammonium salt, (2) analysis of nerve swelling during potassium- and veratridine-depolarization, (3) measurements of nerve swelling associated with repetitive excitation, and (4) studies of ion and water movements across the nerve membrane during potassium depolarization. The importance of water movements during excitation was amply demonstrated by these investigations. Since the time-resolution of our mechanical measurements is very high, we expect to gain a further insight into the process of nerve excitation by accurately determining the time-courses of mechanical changes in the nerve fiber under a variety of experimental conditions.